A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) or an image display device (also simply referred to as a display device). As semiconductor thin films that can be used for the transistors, silicon-based semiconductor materials have been widely known, but oxide semiconductors have been attracting attention as alternative materials.
For example, a technique for forming a transistor using zinc oxide or an In—Ga—Zn oxide semiconductor is disclosed (see Patent Documents 1 and 2).
A high-resolution, low-power display device in which an oxide semiconductor having a c-axis aligned crystal structure is used for an active layer of a transistor is disclosed (see Non-Patent Document 1).
In recent years, demand for integrated circuits in which semiconductor elements such as miniaturized transistors are integrated with a high density has risen with increased performance and reductions in the size and weight of electronic devices.
For example, when a transistor includes, in an active layer, an oxide semiconductor with a crystal structure in which c-axes are aligned in a direction perpendicular to a surface, the transistor has extremely low off-state current (see Non-Patent Document 2). Application of the transistor to an LSI such as a nonvolatile memory, a CPU, or an image sensor by utilizing the features of the transistor is suggested (see Non-Patent Documents 3 to 5).